1. Field of the Invention
The present invention relates generally to signal level detecting circuits for detecting levels of analog signals, and more particularly, to a circuit for detecting the level of an analog signal, such as a variable control signal, and producing a detected output signal which can be digitally processed.
2. Description of Related Art
In a previously proposed circuit for detecting the level of an analog signal and for producing a detected output signal which is formed so as to be digitally processed, it is customary that the analog signal which has a level that is to be detected is subjected to an analog-to-digital (A/D) conversion so as to produce a digital signal which corresponds thereto and the level of the analog signal is indirectly detected based on the digital signal obtained by the A/D conversion. For example, in the case where the brightness of the display of a fluorescent display tube which is used in an electronic apparatus and which is driven by a digitalized driving circuit that is controlled by an analog control signal, the level of the analog control signal is indirectly detected in such a manner that the analog control signal is subjected to an A/D conversion so as to produce a digital signal which corresponds to the level of the analog control signal, and the digital signal obtained during the A/D conversion is supplied to the digitalized driving circuit so as to carry out the brightness control for the fluorescent display tube.
In such a case, it is usual that the A/D conversion is carried out by an A/D converter formed as an integrated circuit and the A/D converter employed in conventional consumer product electronic apparatus is commonly a microcomputer of, for example, the eight bit type.
In the case where the A/D converter contained in the microcomputer of the eight bit type is used for detecting the level of the analog control signal, it is possible that the number of conversion steps for digitalization are insufficient for a sufficient range of level variations in the analog control signal and therefore the level of the analog control signal will be detected with insufficient accuracy. If such insufficiency in the number of conversion steps for digitalization results in the brightness control for a fluorescent display tube, the brightness of the display on the fluorescent display tube does not change smoothly in accordance with variations in the level of the analog control signal but will have unnatural sudden changes in response to the variations in the level of the analog control signal.
For avoiding such a problem, it is considered to use an A/D converter which has more than eight bit numbers for the A/D conversion of the analog control signal. However, A/D converters with more than eight bit numbers require a large number of electronic elements and parts which results in complicated and expensive devices.